Generally, a front derailleur mounts its fixing member by use of a mounting band on a seat tube fixed to the bottom bracket of the bicycle. The linkage members are supported to the fixing member downwardly with respect thereto and carries at the lower ends the movable member, so that the chain guide is positioned below with respect to the fixing member.
Such a conventional front derailleur, when intended to be mounted on a seat tube having a different outer diameter, needs a mounting band of suitable size for mating with every seat tube, whereby many bands of different size must be prepared and connected to the derailleur. As a result, it is very difficult to handle many kinds of front derailleurs which are fit to various seat tubes.
Furthermore, the mounting band, when attached to the seat tube for mounting the derailleur, does not have its vertical or circumferential position defined with respect to the seat tube, whereby the above position must be settled every time the derailleur is mounted, resulting in complicated mounting work.
On the other hand, when a control wire is pulled to swing the linkage members to move the movable member so that the chain is switched from a smaller diameter sprocket to a larger diameter sprocket, the chain guide, which is positioned below the fixing member, moves radially outwardly, that is, upwardly, with respect to the larger diameter sprocket. Hence, the chain during shifting is pressed onto the lateral side of the larger diameter sprocket causing poor speed change efficiency. When the chain is switched from the larger diameter sprocket to the smaller diameter sprocket, the chain guide moves radially inwardly, that is, downwardly, with respect to the smaller diameter sprocket, so that the chain does not disengage smoothly from the larger diameter sprocket, resulting in difficult of quick speed changing.